Akikage

Called the shadow ninja, an akikage is what remains of an eastern assassin killed while stalking an important mark. Obsessed with duty in life, it cannot rest until that last job is finished. Rarely visible, it appears clad all in black, gloved, booted, hooded and masked so nothing human shows. It never speaks but understands any language, and speak with dead gets no answer from it.

Bodiless, it cannot handle objects but glides through solid matter, and it turns invisible at will, giving attackers -4 to hit. It fights unarmed with four blows per round, each doing 1d6 from the deep cold within it.

For its death blow it must become visible for a round and drop its other three attacks. The single punch deals 3d6 cold damage, and the victim must save vs. death magic or die regardless of hit points; a quick autopsy finds the heart frozen solid.

Hitting it requires +2 or better weapons. Sleep, charm, hold and fear spells fail against it, as do poison, disease, and any mental or cold attack. Holy water deals 1d8 per vial. It is turned as a spectre, and either dispel evil or raise dead destroys it.

It wants only its destined victim. Once that kill is made, the curse ends and the spirit dissipates forever.

It needs no food or rest, but every animal, wild or tame, grows nervous within 50 feet of it.

An ansasshia is a shadow ninja enslaved to a living master, usually by an evil priest commanding undead or a control undead spell. Alternatively, someone who learns the target's identity can kill that person first and promptly cast control undead on the corpse, binding the spirit to the caster. Its devotion to duty then keeps it from ever disobeying or betraying its master, however much it longs to be free.